In general, a specific catalyst system is required to produce highly crystalline polypropylene. The catalyst system to produce such polypropylene requires a stereo-regulating agent or external donor. Well known external donors that are used to produce highly stereo regular materials are dicyclopentyldimethoxy silane and diphenyldimethoxy silane. However, these donors significantly reduce the efficiency of hydrogen, which is used as a chain transfer agent in the polymerization. As a result, in a commercial manufacturing facility, the use of these donors limits the polymers produced to low melt flow rates.
U.S. Pat. No. 4,522,994 to Chiba et al describes highly crystalline, high melt flow rate polypropylene produced using a Ziegler-Natta (ZN) catalyst in a slurry process. Although U.S. Pat. No. 4,522,994 claims to have achieved a combination of high crystallinity and high melt flow rate, it does not address the content of xylene solubles in the material as a measure of crystallinity. Further, the slurry process used in U.S. Pat. No. 4,522,994 results in extraction of a significant amount of the solubles produced in the polymerization. Hence, the high total crystallinity of the material is achieved by removing non-crystalline material produced in the process, rather than by controlling its formation in the first instance. This not only reduces the yield from the polymerization, but also creates an additional waste handling issue.
U.S. Pat. No. 6,323,150 to Kojoh et al describes ZN catalysts containing a combination of two internal donors. The first internal donor is either a polycarboxylic acid compound or a polyether. The second internal donor compound is also either a polycarboxylic acid compound or a polyether. The catalysts described in U.S. Pat. No. 6,323,150 are disclosed as being capable of producing olefin polymers of high stereo regularity. However, all of the examples provided in U.S. Pat. No. 6,323,150 describe polymers produced using a slurry process.
Liquid phase polymerization processes can be grouped into two general types; bulk processes and slurry processes. Both the bulk process and the slurry process are described by Ser van der Ven in “Polypropylene and Other Polyolefins”, © 1990, Elsevier Science Publishing Company, Inc., pp. 119-125. Both the slurry process and the bulk process are described as operating with a suspension of growing polymer particles in a liquid. In the case of the slurry process, the liquid is an inert solvent, such as hexane. In the case of the bulk process it is liquid monomer. U.S. Pat. No. 4,522,994 and U.S. Pat. No. 6,323,150 both describe polymers produced in a so-called slurry process.
Another means of achieving a high melt flow rate in a polymer is through post polymerization treatment with a vis-breaking agent to reduce the average molecular weight of the polymer and thus increase the melt flow rate. However, this method necessitates an additional process step and negatively affects the molecular weight distribution Mw/Mn of the polymer. It also creates additional xylene soluble material, which negatively affects the properties of the material.
It would therefore be desirable to provide a process for producing a highly crystalline, high melt flow rate polypropylene using a bulk liquid process. In such a process, the crystallinity, xylene solubles and melt flow rate of the polymer would be controlled by the selection of catalyst and polymerization conditions. Thus the need to extract or wash out solubles, or perform vis-breaking operations would be eliminated.